Clean power

Scientists produce electricity from compressed gas

Published: Wednesday 15 September 2004

scientists at the Bangalore-based Indian Institute of Science (iisc) have found that a jet of gas can generate current if passed over materials like semiconductors. Physicists Ajay Sood and Shankar Ghosh made the discovery by using just a bottle of compressed gas and equipment made at very low cost. The findings, appearing in the Physical Review Letters (Vol 93, No 8, August 20, 2004), have drawn considerable peer attention.

The scientists passed the compressed gases (oxygen, nitrogen and argon) at velocities ranging from a few kilometres per hour to 350 kilometres per hour over various materials -- semiconductors like silicon and germanium, carbon nanotubes, graphite and metals like copper.

Each time they got a measurable current, which falls in the microampere range. The scientists found that the voltage increases with the speed of the gas flow. As per the scientists, though even the maximum current generated was miniscule, it is still a breakthrough as there is no need to burn any fuel to generate it.

The duo along with N Kumar of the Raman Research Institute, Bangalore, had in 2003 demonstrated that the flow of liquids over carbon nanotubes generates a voltage in the tubes. "Having done that we asked ourselves if a similar effect will be produced by passing gases instead of liquids. We found that an equally striking effect exists for gas as well but for entirely different reason," Sood told Down To Earth. According to him, the reasons are linked with an interesting interplay of two well-known physical principles -- first, the pressure differences in the gas flow and second, the flow leading to temperature differences across the base material (Bernoulli's principle), which in turn produces the measured voltage (Seebeck effect).

Sood and Ghosh report that the effect they observed is not restricted to the few materials they studied. Any material with high Seebeck coefficient, such as selenium, tellurium, gallium arsenide and electrically conducting polymers can also produce similar or better results.

One immediate application of the technique is in sensors used for measuring gas flow in equipments of various industries. Currently a gadget called anemometer is used. It works by measuring the changes in heat transfer in the system, and hence a slight change in temperature, pressure and composition of the gas can give erroneous reading.

According to the scientists, the magnitude of current generated can be easily scaled up by connecting millions of these 'nanogenerators' in the same way numerous photovoltaic cells are connected to generate power from the sunlight.

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